1. Field of the Invention
The present invention relates to a focus detecting device of high accuracy for use in a camera or the like, having a plurality of focus detecting areas and capable of correcting an error between an in-focus position of lens detected by focus detecting means and a best image field position.
2. Description of the Prior Art
In a condition in which a plurality of objects are located within an objective field to be photographed, photographers often require to obtain a photograph in which both of a main object and a secondary object located at the background of the main object are reproduced sharply. However, such requirement can not be satisfied by conventional auto-focus camera provided with an automatic focusing device which controls the photographic lens to focus only one object located within the objective field.
To satisfy such requirement, there has been proposed an automatic focusing device in which the focus detection is performed with respect to a plurality of divided areas of the object field, and the focusing condition of the photographic lens is controlled in accordance with a plurality of focus detection results so that all of the detected objects can be reproduced on the image plane within the depth of focus of the photographic lens, or so that a closest object detected can be reproduced sharply on the image plane. The latter is based on a consideration that the main object to be focused is likely to be located at a position closest to the camera among the plurality of detected objects. Such automatic focusing device are proposed, for example, by Japanese Laid-Open Patent Application Nos. 101128/81 and 146028/84.
As well known, however, a photographic lens has various kinds of aberrations, including spherical aberration, astigmatism and field curvature. Beside, due to the influence based on the direction of arrangement of automatic focus detecting elements, when there is made focus adjustment for a photographic lens in accordance with an output signal from a focus detecting element disposed in a position deviated from the central position among many focus detecting elements arranged on the photographing image plane, there occurs a deviation between the in-focus position detected by the automatic focus detecting elements and the best image field position of the photographic lens.
This is as shown in FIG. 1, in which the axis of abscissa X extends along an optical axis, the left-hand side being a photographic lens side and the right-hand side, a film surface side, and the axis of ordinate Y represents the distance from the optical axis.
In FIG. 1, the position indicated as "axial" is a position in which there is obtained the best imaging performance of an image formed by an axial light (incident light parallel to the optical axis of the photographic lens). The said position is usually called "the best axial position". In a camera, however, it is not desirable to locate the film surface in "the best axial position" because the aberrations generated by an off-axial light (incident light having inclination with respect to the optical axis) would be deteriorated. According to the conventional construction, therefore, the film surface is located not in "the best axial position" but in a position slightly deviated therefrom. The aberration curve represented as "image" in FIG. 1 indicates the magnitude of deviation (in full open aperture condition, e.g. F=2.0) of the actual photographic lens transmitted light including both axial light and off-axial light with respect to "the best axial position". It is the best image contrast position.
On the other hand, in an automatic focus detecting device, the focus detection is performed by sensing only the light passing through a portion close to the optical axis of a photographic lens, namely, a portion where the aperture value is large (e.g. F=5.6), so the aberration correction performance on the focus detecting device becomes superior to that of the entire photographic lens. And in the use of a focus detecting sensor (hereinafter referred to as "image sensor"), the focusing condition of the photographic lens is adjusted to a position close to "the best axial position" as indicated as "image sensor stop position" in FIG. 1.
Thus, the lens stop position ("image sensor stop position") detected as an in-focus position by the image sensor deviates from the best image contrast position ("image" position) and this deviation increases as the distance from the optical axis becomes longer.
In view of the above point there has been proposed an automatic focus control device (see Japanese Patent Laid-Open Patent Application No. 208514/84) in which the in-focus position data provided from the image sensor is corrected with spherical aberration data of a photographic lens to correct an error from the best image field position.
The above proposed correction for the in-focus position of a photographic lens intended to correct errors based on the properties peculiar to the photographic lens such as, for example, spherical aberration of the lens. Of course, such correction of errors based on the properties peculiar to the photographic lens is important, but in order to set the photographic lens in the best image position more accurately it has been necessary to decide an optimum correction value according to the focal length of the photographic lens as well as the exposure mode which is determined by the combination of shutter speed and aperture value, and correct the in-focus position of the photographic lens using the said correction value.